Corn is the largest crop, in terms of volume and value, grown in the United States, with production of 228.8 million metric tons during fiscal year 2003. Whole corn kernels can be dry-milled to separate the corn into more useful components. These components can then be ingredients for various food products, and starting ingredients in a variety of non-food manufacturing/processing industries. Most of the dry-milling in the United States is done through the use of a tempering-degerming process. Sieving and aspiration of the tempering-degerming product primarily yields corn endosperm in different size fractions useful for the manufacture of various foods.
Whole corn kernels are used in the manufacture of corn tortillas in which the masa flour used to make the tortillas is prepared by a traditional method called nixtamalization. Tortilla consumption is growing in the United States, as well as in Canada and parts of Europe. The sales of corn tortilla and tortilla chips in the United States in 2000 by large manufacturers totaled $4.4 billion. It is estimated that additional sales of about $2 billion per year can be attributed to smaller tortilla processors and manufacturers.
The traditional nixtamalization process used in tortilla manufacture involves cooking corn kernels, followed by steeping the cooked kernels in an alkaline solution. Generally, the ratio of corn to solution ranges from one part of whole corn kernels to ten parts of the alkaline solution to one part whole corn to three parts alkaline solution. The solution is generally a water and lime solution with 2% by weight lime (CaO). The corn is cooked by boiling followed by a steep period, on the order of 12 hours or more. This cooking process softens the pericarp and allows the endosperm to absorb water, thus facilitating subsequent milling. After steeping, the solution is drained. The steeped corn is called nixtamal, and the solution, or cooking liquor, is called nejayote. Nejayote is a highly alkaline waste product that has a large oxygen demand, and must be disposed of properly, which significantly increases the cost of the traditional nixtamalization process. The nixtamalized kernels are then repeatedly washed with water to remove excess lime and any solubilized particles generating additional waste liquid. The cooked and nixtamalized kernels can be ground or milled, in disk mills for example, with the addition of small amounts of water. The resulting dough is called masa and is suitable for making food products, including tortillas and other related tortilla products such as corn chips, tortilla chips, taco shells and nachos.
Tortillas are made from the masa by forming thin disks of masa with an appropriate diameter (e.g. 12 to 15 cm). These disks are cooked on both sides to obtain the final tortilla product. Generally, tortillas have a relatively short shelf-life in that they quickly harden and lose their flexibility. Traditionally prepared tortillas, without additives, have a maximum shelf life of about 12 to about 15 hours, and after this time they become hard or stale. Therefore, discerning tortilla consumers obtain tortillas manufactured the same day.
Alternatively, the cooked, nixtamalized and milled kernels can be dried so that the end product is a nixtamalized corn flour (also called “instant masa” or “dry masa”). This instant masa can be shipped to tortilla manufacturers or consumers. Water is added to the instant masa to make a reconstituted masa that is used to make tortillas. However, it is generally believed that the instant masa made from the nixtamalized corn flour is inferior to fresh masa, as reflected in the final tortilla product flavor and texture. See Flores-Farias et al. (2000) J. Sci. Food Agric. 80:657-664.
The traditional nixtamalization process has several drawbacks. The process is time and energy intensive, thereby increasing the cost of the final food product. In addition, the process requires a large volume of water and generates large liquid-waste discharge, on the order of three to ten liters of alkaline water solution per kilogram of corn. Alternative technologies for producing instant masa useful in tortilla production have been examined. However, these technologies, including drum drying, micronizing (dry heat treatment), microwave heating, and extrusion have been found to result in an inferior tortilla compared to tortillas produced by traditionally nixtamalized instant masa.
Whole mature corn kernels are comprised of four components: pericarp (hull or bran), germ (embryo), endosperm and tip cap. The germ and endosperm make up approximately 83% and 11% of the whole kernel, by weight. The pericarp, which makes up approximately 5% by weight of whole kernel, surrounds the germ and endosperm. The free oil is located in the germ and the starch in the endosperm. The germ is approximately 35% oil or fat. The starch is in microscopic granular form, surrounded by proteins. The dried component of corn kernel contains approximately 72% starch, 9-10% protein, 4-5% fat or oil and the remainder is fiber, vitamins and minerals. Conventional tortilla composition is, by wet basis, approximately 50% moisture, 8-10% protein, 1.5-2.5% fiber, 1.6-2% fat, 0.8-1.2% ash, 34-38% nitrogen free extract (see U.S. Pat. No. 6,358,550).
As summarized in U.S. Pat. No. 4,594,260 the physical parameters of the nixtamalization process (e.g. water to corn ratio, lime concentration, cooking time and temperature, and washing steps) have been varied in an effort to improve the traditional nixtamalization process. More recent efforts have examined the effects of fractionating the corn kernel prior to nixtamalization thereby decreasing the volume of effluent waste. See e.g. U.S. Pat. Nos. 4,594,260, 6,265,013 and 6,358,550. However, these efforts have focused on nixtamalization of the pericarp fraction.
U.S. Pat. No. 4,594,260 separates the corn into pericarp, germ and endosperm fractions, with only the pericarp fraction nixtamalized. After pericarp nixtamalization the germ and endosperm fractions are mixed with the nixtamalized pericarp. The '260 patent reports that a main factor causing brittleness of tortillas manufactured from corn flour is that the entire corn kernel is subjected to an accelerated nixtamalization process. The traditional nixtamalization process ensures a uniform hydration of the starch contained within the endosperm, thereby avoiding excessive gelatinization and resulting in tortillas with good softness and flexibility characteristics. The '260 patent concludes that the nixtamalization of endosperm is unnecessary and undesirable and, therefore, only the pericarp should be nixtamalized. The other fractions (germ and endosperm) are instead subjected to a hydration step. It was reported that hydrated, but not gelatinized, starch yields soft, flexible and formable tortillas with excellent folding characteristics.
U.S. Pat. No. 6,265,013 separates the corn into pericarp, germ and endosperm fractions and selectively nixtamalizes the pericarp fraction. The germ and endosperm fractions are hydrated with water, and all fractions are then mixed back together to produce fresh masa or nixtamalized corn flour. Thus, the end product essentially contains the entire germ from the original corn kernel.
U.S. Pat. No. 6,383,547 discloses the use of a by-product of cereal milling as an additive to increase the strength and/or shelf life of tortillas and related products made from masa. In particular, the grain by-product is the hull or pericarp, which contains starch that has been gelatinized by cooking it in water containing an alkaline agent. The corn grain is nixtamalized separately from the corn pericarp and then mixed back together to form masa.
U.S. Pat. No. 6,358,550 separately nixtamalized the grain and the pericarp, and then mixed the grain/pericarp in a proportion of 40-45:60-55 to make dietetic corn tortillas containing conventional protein content (8-9%), with half the fat (0.7-1%), and triple the fiber (6-9%).
Without wishing to be limited to any particular theory, it is believed that the alkaline treatments and grinding modify the structure and properties of corn starch. Researchers (Gomez et al, 1990; Gomez et al, 1992) have reported the role of various corn components during nixtamalization. It has been proposed that alterations in starch crystallinity caused by cooking arise from partial starch gelatinization, limited granule swelling and disruption of the crystalline structure. During steeping, more ordered starch structures are formed, with recrystallization or annealing, continued granule swelling throughout the endosperm, and starch solubilization increases. In addition, grinding or milling of the cooked and steeped corn (the nixtamal) releases and disperses swollen starch granules.
The masa obtained from the nixtamalization process is a mixture comprising starch polymers, mixed with partially gelatinized starch granules, intact starch granules, pieces of endosperm and lipids. All of these components develop a complex heterogeneous network in a continuous water phase (Gomez et al, 1987). Additionally, the time-temperature-dependent reassociation of dispersed amylose and amylopectin continuously modifies total water content (Pflugfelder et al, 1988) and its distribution within the network.
Progress has been made in understanding nixtamalization and its effect on corn. Rooney & Suhendro (1999) suggest that lime acts on the cell wall and converts hemicelluloses into soluble gums. In the same way, the alkali-temperature treatment can gelatinize the starch and saponifies part of the lipids, releasing niacin from the niacytin complex and solubilizing a portion of the protein that surround the starch granules. Additionally, owing to the high pH, the glucan chains from amylase and amylopectin are charged, which can help slow retrogradation and improve the freshness of tortillas.
To accomplish these chemical changes and produce high-quality masa, optimal cooking and grinding conditions are believed to be required because of the importance of gelatinization levels. Small amounts of starch are gelatinized during cooking and steeping. Most gelatinization is due to attrition during subsequent corn grinding, which also disperses partially swollen granules into a matrix that act as glue, holding the masa particles together. Too much gelatinized starch, due to overcooking, produces stickiness, making the handling of masa more difficult. On the other hand, undercooking results in a non-cohesive masa that produces tortilla of poor texture; the grinding by itself cannot be used to gelatinize the starch in severely undercooked maize. Rooney & Suhendro, (1999).
Bryant & Hamaker (1997) characterized the influence of lime concentration on the gelatinization properties of maize flour. These authors reported that swelling-power, solubility, and degree of gelatinization can increase at low lime levels (<0.2%, w/v) and then decrease with increasing lime concentration. According to Martinez-Bustos et al (1998), starch crystallinity of corn meal extrudates increases when adding up to 0.15% (w/v) lime, and decreases if additional lime is added. Rodriguez et al (1996) showed that crystallinity and thermal diffusivity of corn tortillas change with lime concentration, both properties reached a maximum at a concentration of 0.2% (w/v) and tend to decrease at higher lime concentrations. However, Modragon et al, (2004) reported no effect of lime on thermal properties using differential scanning calorimetry (DSC); in general, the calcium ions provided by the lime acted as a factor for structural disorganization.
Arambula-Villa et al (2001), and Fernandez-Munoz et al (2002) have reported the importance of steeping time on the nixtamalization process. Steeping times of 4 and 7-9 h, respectively, were found sufficient to produce good quality tortillas. Cooking time is also an important factor to attain optimal nixtamal quality. The texture of fresh corn masa is affected by cook time when all other conditions are kept constant (Ramirez-Wong et al, 1994). It has been reported that the quality of tortillas depends on the method used for preparing the flour or masa (Bedolla, 1983, Arambula-Villa et al, 2001). Bedolla (1983) and Arambula-Villa (2001) report that this is due to the chemical and physical interactions that occur among the different components of corn grains (starch, lipids, fiber and proteins) and lime during the cooking process. This changes the microstructure of flour and masa, and changes their physicochemical, rheological and textural properties (Rodriguez et al, 1996). However, it has been reported that it is important that certain compounds are released from nixtamalized pericarp (gums) and nixtamalized germ (saponified lipids) in order to positively affect overall quality of the masa and tortillas in terms of rheological properties (Martinez-Bustos et al. 2001). Therefore, Martinez-Bustos et al. (2001) suggests adding the nixtamalized pericarp and germ components back together with the nixtamalized endosperm fraction.
It is generally recognized that corn physical characteristics are important factors that affect the end product obtained from the nixtamalization process. Nixtamalization variables can be manipulated, and their effect on masa and tortilla properties measured, to obtain optimal nixtamalization process variable values that produce an acceptable masa texture (Sahai et al, 2000). Sahai et al. reported that product variables such as masa texture and tortilla color were influenced not only by processing parameters (cook temperature, cook time, and steep time), but were also dependent on the initial raw material corn characteristics. Reyes-Moreno et al. (2004) used response surface methodology to report the optimum combination of nixtamalization process variables for the production of nixtamalized corn flour from whole corn kernels (quality protein maize (QPM)) (31 minutes cook time, 5.4 g Ca(OH)2/l, and 8.1 hours steep time). A better understanding of the role of the components of nixtamalized corn and their effect on the quality of the masa and tortillas is important in order to develop processes that can improve the traditional tortilla-making process.
It has been suggested that undue gelatinization of the corn starch contained in the endosperm, and associated adverse rheological changes, adversely affects the end food products (e.g. tortillas). U.S. Pat. No. 4,594,260; Martinez-Bustos et al. (2001) J. Sci. Food Artic. 81:1455-62. Therefore, fractionation methods generally focus on nixtamalization of the pericarp rather than the endosperm, and do not manufacture a tortilla solely from the endosperm fraction. Martinez-Bustos (2001) (“Tortillas from nixtamal with the germ removed showed the worst texture, rollability and puffing.”). Martin-Martinez et al. (2003) employed selective nixtamalization of two fractions, an endosperm fraction and a pericarp, germ, and tip cap (PGT) fraction, using response surface methodology to manufacture tortillas that had similar properties to those prepared by the traditional nixtamalization process. They reported, however, that “[t]ortillas of good functional characteristics similar to tortillas produced by the traditional process were obtained when 5% nixtamalized fractions of PGT were blended with 95% nixtamalized endosperm.”
There is a continuing need in the art for improving the masa-making process, to decrease waste effluent and increase cooking efficiency while retaining or improving tortilla quality. This invention addresses these needs.